National Examination for 2013 IChO
Taiwan
Round 4 Practical Examination (5 hours)
Task 1. Enolate Condensation: Synthesis of dibenzalacetone
A. Principle
The Claisen-Schmidt reaction involves the synthesis of anα,β-unsaturated ketone by the
condensation of an aromatic aldehyde with a ketone. The aromatic aldehyde possesses no
hydrogens α-to the carbonyl group, it cannot therefore undergo self condensation but reacts rapidly
with the ketone present.
The initial aldol adduct cannot be isolated as it dehydrates readily under the reaction conditions
to give an α,β-unsaturated ketone. This unsaturated ketone also possesses activated hydrogens
α-to a carbonyl group and may condense with another molecule of the aldehyde.
In this experiment you will carry out the base catalysed aldol condensation of benzaldehyde with
acetone. The product will be purified by recrystallization.
H3C
Ο
O
Ο
CH3
+
Acetone
molar mass: 58 g/mol
density: 0.79g/mL
limiting reagent
2
H
NaOH, H2O
C2H5OH
Benzaldehyde
molar mass: 106 g/mol
density: 1.04 g/mL
an excess will be used
Dibenzalacetone
(1,5-diphenyl-1,4-pentadien-3-one)
molar mass: 234 g/mol
B. Reagents
acetone, benzaldehyde, 95% alcohol, NaOH solution (3 M), ethylacetate, and hexanes.
C. Equipments
Test tube (15 x 2 cm), test tube rack, test tube clamp, septum, beaker (100 mL), graduate cylinder
(10 mL), droppe, watch glass, spatular, syringe and needle (1 mL), microcentifuge tube (1.5 mL),
TLC and TLC chamber.
Hot water: 60℃, vacuum filtration, filter paper and funnel, melting point instrument.
D. Procedure
Notice: benzaldehyde should not be more than 2 equivalents.
1. Take 4 mL 3 M NaOH solution and 3.2 mL of 95 % ethanol into large test tube, mix well.
2. Add 0.5 mL of benzaldehyde, and then 0.15 mL of acetone into the test tube. Use septum to stop
the test tube to avoid vaporization of acetone.
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3. Shake the test tube to mix solution. Hold the septum to prevent gas leak out.
the test tube.
15 min.
Continuously shake
Yellow solid will form in about 3~5 min. Shake the test tube occasionally for about
Collect the solid by vacuum filtration. Wash the solid with water.
4. Transfer the solid into a small test tube, and pour in 4 mL of 95% ethanol.
Use test tube clamp
and put the test tube in 60℃ water bath until all solid dislove.
5. Take the test tube out and cool to room temperature.
Leave the test tube on ice bath for about 15
min to crystallize.
6. Weight out a filter paper then do vacuum filtration.
Wash the solid with small amount of ice cold
ethanol.
7. Transfer the filter paper with the yellow products onto a watch glass and put it into 50℃ oven for
drying.
8. Weight out the products, calculate the yield.
Measure melting point.
9. Take small amount of product into microcentifuge tube.
Use hexanes:ethylacetate = 10:1 to elude the TLC.
2
Disolve it in ethylacetate and run TLC.
Calculate Rf.
Task 2. Quantitative analysis of Ascorbic Acid in Vitamin C Tablet
The main component of over-the-counter vitamin c tablet is ascorbic acid (MW=176.12). It
is an acid and a reductant. Both acid-base and redox titration can be employed to quantitatively
determine the amount of ascribe acid.
This experiment has two parts, the first part is acid-base titration and the second part is redox
titration.
You are asked to give a command on these two types of titration in this application.
Your points are base on the accuracy of the amount of unknown sample.
gets full points.
Error between 2 to 7 % will scale linearly to the full points.
Error within 2 %
Error larger than 7
% gets no points.
Check your reagents and equipments before you start
Reagent
NaOH(aq)
1 bottle
(concentration shown on label)
Equipment
Calculator 1 ea
Graduated cyclinder：
~0.01 M I2(aq)
1 bottle
(concentration shown on label)
50 mL
10 mL
Indicator
Erlenmeyer：
phenolphthalein
methyl red
starch
125 mL
Beaker：
1000 mL
100 mL
Miscellaneous
1 ea
1 ea
5 ea
1 ea
3 ea
Volumnmetric：
Distilled water
Kimwipe
Detergent
100 mL
25 mL
1 ea
1 ea
10 mL
1 ea
2 x 50 mL Burette and rack
safety goggle
1 pair
glove
napkin
1 pair
1 ea
funnel
dropper
glass rod
3
1 ea
5 ea
1 ea
Procedure
1. Dissolve a tablet of Vitamin C provided into about 60 mL of water, then diluted to 100 mL
precisely.
A: Acid-Base titration
1. Pipette 10 mL of the above Vitamin C solution into an Erlenmeyer flask.
Add few drops of
proper indicator before titrate with NaOH solution with known concentration.
2. Repeat step 1 at least three times.
Take an average of the titrated volume.
B: Oxidation-reduction titration
1. Pipette 10 mL of the above Vitamin C solution into an Erlenmeyer flask. Use standard iodine
solution to titrate and use starch as indicator. The blue color must remain at least 15 seconds at the
end of titration.
2.
Repeat step 1 at least three times.
Take an average of the titrated volume.
The solution provided in this experiment are all standardized with the concentration written on the
reagent bottle. Use the given concentration for all calculation.。
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Task 3. The Menshutkin Reaction
The nucleophilic substitution reaction between a tertiary amine and an alkyl halide is known as
the Menshutkin reaction. This experiment investigates the rate law for the reaction between the
amine known as DABCO (1,4-diazabicylo[2.2.2]octane) and benzyl bromide:
N
N
N
+
Ph
Br
+
N
DABCO
Br
Ph
It is possible for the second nitrogen in the DABCO molecule to react with a second benzyl
bromide. However, in this experiment the DABCO will always be in excess so further reaction is
unlikely. The reaction could proceed by either the SN1 or the SN2 mechanism. In this
experiment, you will confirm that the order with respect to benzyl bromide is 1 and determine the
order with respect to DABCO. This should enable you to distinguish between the two possible
mechanisms.
As the reaction proceeds neutral species, DABCO and benzyl bromide, are replaced by
charged species, the quaternary ammonium ion and Br. Therefore the electrical conductivity of
the reaction mixture increases as the reaction proceeds and so the progress of the reaction can be
followed by measuring the electrical conductivity as a function of time.
Benzyl bromide is a lacrymator.
This experiment should be performed in a fume hood.
The Method in Principle
The rate law for the reaction can be written as
d[Br - ]
= k[RBr][DABCO]α
dt
[1]
where we have assumed that the order with respect to the benzyl bromide, RBr, is 1 and the order
with respect to DABCO is α.
In the experiment, the concentration of DABCO is in excess and so does not change
significantly during the course of the reaction. The term k[DABCO]α on the right-hand side of
Eqn. [1] is thus effectively a constant and so the rate law can be written
d[Br - ]
= k app [RBr] where k app = k[DABCO]α
dt
[2]
kapp is the apparent first order rate constant under these conditions; it is not really a rate "constant",
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as it depends on the concentration of DABCO.
To find the order with respect to DABCO we measure kapp for reaction mixtures with different
excess concentrations of DABCO. From Eqn. [2], and taking logs, we find
ln k app = ln k + α ln[DABCO]
[3]
So a plot of ln kapp against ln [DABCO] should give a straight line of slope α.
kapp may be found by measuring the conductance at time t, G(t), and at time infinity, G∞. In
the supplementary material it is shown that a graph of ln[G∞ –G(t)] against t should be a straight
line with slope kapp.
In practice it is rather inconvenient to measure conductance at time infinity but this can be
avoided by analysing the data using the Guggenheim method.
In this method each reading of the
conductance at time t, is paired up with another at time t + ∆, G(t+∆), where ∆ is a fixed time
interval that needs to be at least a half-life. As shown in the supplementary material, a plot of
ln[G(t+∆)–G(t)] against time should be a straight line of slope –kapp.
For example, suppose we
take measurements at fixed regular intervals, say each 30 s and choose an appropriate value of ∆,
say 3 minutes (180 s). The plot made is of the points {x,y} = {0, ln[G(180)-G(0)]},
{30, ln[G(210)-G(30)]}, {60, ln[G(240)-G(60)]}, ...
The Apparatus
Cheap conductivity meters are commercially available, for example the Primo5 conductivity stick
meter from Hanna instruments works well with this practical. These simply dip into the solution
and the conductance of the solution can be read off the digital display.
Procedure
You are provided with the following solutions, all in ethanol: 0.15, 0.20 and 0.25 mol dm–3
DABCO, and approx. 0.6 mol dm–3 benzyl bromide (this must be freshly made up). You should
measure kapp for each of these solutions by measuring the conductance as a function of time and
then analysing the data using the Guggenheim method. From the three values of kapp, the order
with respect to DABCO can be found by plotting ln kapp against ln [DABCO], as shown by Eqn.
[3].
Ideally we ought to keep the reagents and the reaction mixture in a thermostat. However, as the
heat evolved is rather small, the temperature will remain sufficiently constant for our purposes.
Kinetic Runs
1.
Rinse the conductivity dipping electrode with ethanol from a wash bottle, catching the waste in
a beaker.
Allow the excess ethanol to drain off and gently dry the electrode with tissue.
2.
Transfer 10 cm3 of the DABCO solution to a clean dry boiling tube.
3.
4.
Add 100 µl of the benzyl bromide solution.
Insert and withdraw the dipping electrode of the conductance meter a few times in order to mix
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the solution and then, with the electrode in place, start the stop-watch.
5. Record the conductance at 30 second intervals (it is essential to make the measurements at
regular intervals), starting with the first reading at 30 seconds and continuing until there is no
further significant change in the conductance, or for 10 minutes, whichever is the shorter time.
6. From time to time, gently lift the electrode in and out so as to stir the solution.
7. Once the measurements have been made, remove the electrode, discard the solution and clean
the electrode as in step 1.
8.
Make the measurements for the 0.15 mol dm–3 solution of DABCO, and then for the 0.20 and
0.25 mol dm–3 solutions.
Data Analysis
For each run determine kapp using the Guggenheim method – three minutes is about right for the
fixed interval ∆. Then plot ln kapp against ln [DABCO] and hence determine the order with
respect to DABCO.
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Supplementary information
The key to this experiment is how to use the measured conductance of the reaction mixture to
determine the first order rate constant, kapp. The first stage is simply to integrate the rate law; to
do this we note that for each benzyl bromide molecule that reacts one bromide ion is generated so
that at any time [Br–] = [RBr]init – [RBr], where [RBr]init is the initial concentration of benzyl
bromide.
Thus the rate equation can be written in terms of [Br–] by putting [RBr] = [RBr]init –
[Br–]; integration is then straightforward:
d[Br - ]
= k app [RBr]init − [Br - ]
dt
d[Br - ]
∫ [RBr]init − [Br - ] = ∫ kapp dt
(
)
(
i.e.
)
(
)
− ln [RBr]init − [Br - ] = kappt + const.
The constant can be found by saying that at time zero, [Br–] = 0, hence
− ln ([RBr]init ) = const.
hence
(
)
− ln [RBr]init − [Br - ] = k app t − ln([RBr]init )
which can be written
[Br - ] = [RBr]init (1 − exp[− k app t ]) [4]
When the reaction has gone to completion, at time infinity, the concentration of bromide is equal to
the initial concentration of RBr so Eqn. [4] can be written
[Br - ] = [Br - ]∞ (1 − exp[−k app t ])
-
where [ Br ]∞ is the concentration of Br– at time infinity.
[5]
Equation [5] says that the
-
concentration of Br– approaches a limiting value of [ Br ]∞ with an exponential law.
relationship can be written for the other product, the quaternary ammonium ion, whose
+
concentration will be written [ R 4 Br ] .
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A similar
[R 4 Br + ] = [R 4 Br + ]∞ (1 − exp[−k app t ]) [6]
We will assume that the conductance of the reaction mixture, G, is proportional to the concentration
of the charged species present:
G = λBr- [Br - ] + λR
+
4N
[R 4 N + ]
where λ are simply the constants of proportionality.
Using Eqns. [5] and [6] to substitute for the concentration of Br– and R4N+ we find
G = λBr- {[Br −]∞ (1 − exp[− k app t ])} + λR
(
= λBr - [Br −]∞ + λR
+
4N
)
{[R Br ] (1 − exp[−k
+
+
4N
∞
4
(λ
Br -
[7]
[Br −]∞ + λR
+
4N
)
[R 4 Br + ]∞ is the conductance at time
infinity, G∞.
Equation [7] can be rearranged to give a straight line plot:
1−
G
= exp[−k app t ]
G∞

G 
 = − k app t
ln1 −
 G∞ 
or
)}
[R 4 Br + ]∞ (1 − exp[− k app t ])
= G∞ (1 − exp[−k app t ])
where we have recognised that
app t ]
or
G −G
 = − k app t
ln ∞
 G∞ 
ln(G∞ − G ) = −k app t + ln G∞
Hence a plot of ln (G∞ − G ) against t should be a straight line with slope kapp.
The Guggenheim Method
From Eqn. [9] the conductance at time t, G(t), can be written
G (t ) = G∞ (1 − exp[−k app t ])
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At some time (t + ∆) later the conductance is G(t + ∆)
G (t + ∆ ) = G∞ (1 − exp[−k app (t + ∆)])
The difference G(t + ∆) – G(t) is
G (t + ∆ ) − G (t ) = G∞ (1 − exp[−k app (t + ∆ )] − 1 + exp[− k app t ])
= G∞ (exp[−k app t ] − exp[− k app (t + ∆ )])
= G∞ exp[− k app t ](1 − exp[−k app ∆ ])
Taking logarithms of both sides gives, from the last line,
ln(G (t + ∆ ) − G (t ) ) = ln G∞ − k app t + ln (1 − exp[−k app ∆ ])
This implies that a plot of ln (G (t + ∆) − G (t ) ) against time should be a straight line of
slope –kapp; to make this plot there is no need to know the value of the conductance at infinite
time, G∞, and this is the main advantage of the Guggenheim method.
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Answer sheets
Task 1 (70 points)
product
Attach TLC
Dibenzalacetone
molecular weight
# of mole
theoretical weight
(g)
experimental
weight (g)
filter paper:
g
filter paper + product:
g
product:
g
yield (50 pts)
%
melting point
(10 pts)
°C
R f (10 pts)
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Task 2 (100 points)
A: Acid-base titration (40 pts)
1st titration:
sample
mL,
NaOH
mL.
2nd titration:
sample
mL,
NaOH
mL.
3rd titration:
sample
mL,
NaOH
mL.
sample
mL,
NaOH
mL.
sample
mL,
NaOH
mL.
Average volume for NaOH solution
mL。
B: Oxidation-reduction titration (40 pts)
1st titration:
sample
mL,
NaOH
mL.
2nd titration:
sample
mL,
I2 solution
mL.
3rd titration:
sample
mL,
I2 solution
mL.
sample
mL,
I2 solution
mL.
sample
mL,
I2 solution
mL.
Average volume for I2 solution
mL。
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C: Calculation:： (4 pts for each question)
1. Assume ascorbic acid is a monoprotic acid, calculate the total amount of ascorbic acid in the
Vitamin C tablet from acid-base titration.
2. Write down the balance equation of iodine and thiosulfate reaction.
3. Write down the balance equation of iodine and Vitamin C reaction.
4. Calculate the total amount of ascorbic acid in the Vitamin C tablet from both acid-base and redox
titration.
D: Are the amount of ascorbic acid from two titrations the same? Why is that?
titration methods. List the advantage and disadvantage of both methods.
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Compare the two
Task 3 (100 points)
1. Deduce kapp.。Two graphs for each concentration, one for experimental conductivity vs time and
one for processed plot (They may be plotted on the same graph paper with clear label). Label
concentration on each graph paper and write your name on it. (25 pts for each concentration)
kapp (with unit)
[DABCO]
0.15 M
0.20 M
0.25 M
2. The order of DABCO in the rate law (need plot too). (15pts)
3. Write down the rate law and calculate the rate constant. (10 pts)
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